Virus structure and classification

A major branch of virology is virus classification. Viruses can be classified according to the host cell they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages (viruses infecting bacteria, which include the most complex viruses).

Another classification uses the geometrical shape of their capsid (often a helix or an icosahedron) or the virus's structure (e.g. presence or absence of a lipid envelope). Viruses range in size from about 30 nm to about 450 nm, which means that most of them cannot be seen with light microscopes. The shape and structure of viruses has been studied by electron microscopy, NMR spectroscopy, and X-ray crystallography.

The most useful and most widely used classification system distinguishes viruses according to the type of nucleic acid they use as genetic material and the viral replication method they employ to coax host cells into producing more viruses:
Mimivirus. Mimivirus is a viral genus containing a single identified species named Acanthamoeba polyphaga mimivirus (APMV).

It also refers to a group of phylogenetically related large viruses, designated usually "MimiN.
"[1] In colloquial speech, APMV is more commonly referred to as just "mimivirus.
" Mimivirus, short for "mimicking microbe," is so called to reflect its large size and apparent Gram-staining properties.[2] Mimivirus has a large and complex genome compared with most other viruses. Discovery[edit] The same team that discovered the mimivirus later discovered a slightly larger virus, dubbed the mamavirus, and the Sputnik virophage that infects it.[5]
Prion. A prion ( A protein as an infectious agent stands in contrast to all other known infectious agents, like viruses, bacteria, fungi, or parasites—all of which must contain nucleic acids (either DNA, RNA, or both).

All known prion diseases in mammals affect the structure of the brain or other neural tissue and all are currently untreatable and universally fatal.[4] While PrP is considered the only mammalian prion, prion-like domains have been found in a variety of other mammalian proteins. Some of these proteins have been implicated in the ontogeny of age-related degenerative disorders such as amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U), Alzheimer's disease, and Huntington's disease.[5] This has given rise to the 'prion paradigm', where otherwise harmless proteins can be converted to a pathogenic form by a small number of misfolded, nucleating proteins.[6] Prion Protein (PrP)[edit]

Satellite (biology)
A satellite is a subviral agent composed of nucleic acid that depends on the co-infection of a host cell with a helper or master virus for its replication.

When a satellite encodes the coat protein in which its nucleic acid is encapsidated it is referred to as a satellite virus. A satellite virus of mamavirus that inhibits the replication of its host has been termed a virophage.[1] However, the usage of this term remains controversial due to the lack of fundamental differences between virophages and classical satellite viruses.[2] The genomes of satellites range upward from 359 nucleotides in length for Satellite Tobacco Ringspot Virus RNA (STobRV).[3] Satellite viral particles should not be confused with satellite DNA.

Viroid. Viroids are the smallest infectious pathogens known, consisting solely of short strands of circular, single-stranded RNA without protein coats.

They are mostly plant pathogens, some of which are of economical importance. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleobases.[1] In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleobases in size. The human pathogen hepatitis D virus is a defective RNA virus similar to viroids.[2] Viroids, the first known representatives of a new domain of "sub-viral pathogens", were discovered, initially characterized, and named by Theodor Otto Diener, plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, in 1971.[3][4] The first viroid to be identified was Potato spindle tuber viroid (PSTVd). Some 33 species have been identified.

Taxonomy[edit] Putative secondary structure of the PSTVd viroid Transmission[edit]
DNA virus. A DNA virus is a virus that has DNA as its genetic material and replicates using a DNA-dependent DNA polymerase.

The nucleic acid is usually double-stranded DNA (dsDNA) but may also be single-stranded DNA (ssDNA).
RNA virus. An RNA virus is a virus that has RNA (ribonucleic acid) as its genetic material.[1] This nucleic acid is usually single-stranded RNA (ssRNA), but may be double-stranded RNA (dsRNA).

Notable human diseases caused by RNA viruses include Ebola hemorrhoragic fever, SARS, influenza, hepatitis C, West Nile fever, polio, and measles. The ICTV classifies RNA viruses as those that belong to Group III, Group IV or Group V of the Baltimore classification system of classifying viruses, and does not consider viruses with DNA intermediates in their life cycle as RNA viruses.[3] Viruses with RNA as their genetic material but that include DNA intermediates in their replication cycle are called retroviruses, and comprise Group VI of the Baltimore classification. Notable human retroviruses include HIV-1 and HIV-2, the cause of the disease AIDS.
DsDNA-RT virus.